1. Field of the Invention
The present invention relates to a dust seal structure for a valve stem in a rotary throttle valve carburetor.
2. Description of the Related Art
There are rotary throttle valve carburetors that have a cylindrical throttle valve provided with a metering needle and a throttle through-hole, and disposed perpendicularly with respect to an air intake passage in a main body of a general-purpose carburetor used as a power source in small vehicles and portable machinery for agriculture and forestry applications. The carburetors also have a fuel nozzle disposed on the central axis of the throttle valve, made to open into the throttle through-hole, and provided with the metering needle inserted therein. The valve stem of the throttle valve, which protrudes at one end from the shaft hole formed in the cover body of the throttle hole in the carburetor body, is caused to rotate in accordance with the operation of the accelerator pedal, whereby the throttle valve moves in the direction of its central axis while rotating as a whole, and controls the airflow rate and the fuel flow rate.
Since the throttle valve in the rotary throttle valve carburetor described above moves in the axial direction and controls the airflow rate and the fuel flow rate while rotating as a whole, any foreign matter that passes through the gap between the valve stem and the shaft hole formed in the cover body and that enters the operating part of the valve stem of the throttle valve, the cam mechanism that causes the throttle valve to move in the axial direction, or other components can prevent the smooth operation of a throttle valve lever, and the performance of the engine may be reduced if air leaks through the gap. A dust-proofing member is fitted in order to block the gap formed between the valve stem and the shaft hole formed in the cover body.
There is known in the prior art a technique whereby a cylindrically shaped dust-proof member formed from rubber or another elastic material having adhesive properties is disposed between an operating lever attached to a distal end of a valve stem and the cover body. However, as described above, the valve stem moves in the direction of its central axis while rotating together with the throttle valve to control the airflow rate and the fuel flow rate. Particularly in an idling state, the throttle valve is positioned at the lowermost point in the axial direction, and the distance between the operating lever and shaft support part is at a minimum. If the dust-proof member is kept in this state for a prolonged period of time, it will consistently be placed under crushing pressure, causing it to degrade and deform.
A problem has accordingly been presented in that the dust-proof member first extends fully in the axial direction but then becomes incapable of maintaining adequate elasticity at the extended length, causing the part anchored to the cover body side to detach and the dust-proof effect to be compromised, when the dust-proof member degrades and deforms, and the throttle valve ascends with increased engine speed, and particularly when the engine speed increases, the throttle fully is open, and the maximum amount of fuel is injected.
A dust seal structure for a valve stem in a rotary throttle valve carburetor that overcomes the above problem is disclosed in the Japanese Laid-open Patent Publication No. 2005-146980.
In the disclosed dust seal structure, a dust-proof seal 3a composed of an elastomer material for dust-proofing the gap between the valve stem 1a of the throttle valve and the cover member 2a for supporting the valve stem is formed in the shape of a cylinder that is widened at the center, an outward-facing flange part 4a is provided in the central region of the valve stem 1a, and the dust-proof seal 3a is caused to engage with and become attached to the valve stem 1a and the cover member 2a at three points; i.e., at either end of the dust-proof seal 3a and at the central widened part, as shown in FIG. 4.
This dust seal structure for a valve stem can maintain its dust-proofing properties more reliably than can the prior art model involving engagement at two points; i.e., the upper and lower end. Since the two ends of the dust-proof seal are reduced in diameter relative to the central part, the open end on the shaft support side can be prevented from moving past the part projecting outward in a radial direction, and a state can be assumed in which the gap between the turning support shaft and the shaft support is covered at all times by the dust-proof seal. The dust-proofing function of the dust-proof seal is improved along with durability, dispensing with the need to replace the dust-proof seal.
However, the elastomeric dust-proof seal disclosed in the above patent publication is disposed on the outer perimeter of the valve stem so as to be exposed to ambient air, and degrades under ultraviolet radiation from sunlight, exhaust emissions, and other aspects of the operating environment while in use. The repeated episodes of extending and contracting in accompaniment with the throttle valve moving in the axial direction during use also causes degradation, so that the dust-proof function is compromised, and the dust-proof seal must be replaced. Accordingly, problems are presented in that maintenance costs accrue, or inspections increase in complexity. A further problem is presented in that the length of the valve stem that protrudes from the cover body must be at least the height of the dust-proof seal, and it is not possible to reduce the mounting dimension in the height direction.